The mouse formin gene when mutated gives rise to a recessively- inherited dysmorphic syndrome consisting of oligosyndactyly with synostosis of the long bones of the limbs and renal agenesis/dysgenesis. Several protein isoforms are encoded by this gene. Six mutant alleles have been described. These mutants exhibit a completely penetrant limb phenotype and incompletely penetrant renal agenesis (20-98%). An isoform-specific null exhibits only the renal phenotype. We found that the variable pentrance is allele-specific. Given the complexity of the gene structure and the mutant phenotype, this gene?s function will be studied by generating a complete deletion. In addition, crosses of different formin mutants with Os, another mouse mutant with an acrorenal syndrome, have revealed genetic interactions between Os and Fmn.